topographic highs. By studying cores and comparing them with onshore 

 geology, it appears that the acoustic basement (as defined in this study) 

 is not everywhere coincident with the top of pre-Cretaceous basement 

 complex rocks but includes overlying glacial deposits in places. Willett, 

 et al . (1972) and Setlow (1973) found generally similar reflection patterns 

 in the subbottom of the study area. 



The surface of the acoustic basement is highly irregular and complex 

 (Fig. 6) . Most of the surface appears to be a combination of two topo- 

 graphic elements: (a) a broad primary system of highs and lows with 

 relief up to 200 feet; and (b) a complex series of secondary relief fea- 

 tures with local relief often exceeding 50 feet. The secondary features 

 are profuse and several may appear in the seismic reflection record from 

 a single survey mile. Thus, many secondary relief features may lie undis- 

 covered between survey tracklines, and detailed delineation of surface 

 topography on the acoustic basement is not feasible with available data. 



The primary topography on the acoustic basement was deliniated by 

 smoothing the profiles so that slope trends were projected across the 

 base of secondary highs (Fig. 7) . The surface contours in Figure 8 are 

 based on the smoothed data; the map is judged to be a reasonable rendition 

 of the primary basement topography because elevations derived from the 

 smoothing process, when plotted and contoured, reveal a coherent pattern 

 of highs and lows with consistent directional trends. 



The primary topography consists mostly of elongate lows and elongate 

 to equidimensional or irregular highs. The orientation of primary features 

 is east to east-southeast with some of the linear depressions closed or 

 partly closed. If these depressions are remnants of an ancient drainage 

 system (as seems most likely), then it appears the system has been dis- 

 rupted. A probable reason for this is that acoustically impenetrable 

 glacial deposits fill parts of the valleys, and that many of the innu- 

 merable secondary topographic highs are also glacial features, 



c. Transparent Reflection Unit . Commonly the transparent unit is 

 separated into upper and lower subunits by an irregular reflecting surface 

 characteristically producing a strong, well defined signal (Fig. 9). This 

 reflector is called the blue reflector, a conventional term used in other 

 ICONS studies to designate the uppermost strong reflector in subbottom 

 strata. The configuration of the blue reflector indicates that it is 

 probably an erosional surface. 



Below the blue reflector, the transparent unit characteristically 

 has one of two aspects on ICONS reflection profiles: (a) a reflectively 

 featureless unit with uniform mottled-gray tone; or (b) an internal 

 reflector pattern consisting of weakly defined, closely spaced reflectors 

 lying in a horizontal or gently warped attitude. 



In the subunit lying above the blue reflector, internal reflections 

 are generally much stronger, are often discontinuous, and may lie at 

 relatively steep angles to the horizontal. These reflector patterns 



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